Hubless cast iron soil pipes and fittings were developed in the early 1960's and the pipes were connected using a stainless steel and elastomer construction as shown in the Canadian Government Specification Board 77-GP-2 of Dec. 8, 1961 and the Cast Iron Soil Pipe Institute (Washington, D.C.) Standard No. 301-64T in 1964. The design set out in the standards is still the predominant design used in hubless systems throughout North America.
An ideal soil pipe coupling has the following characteristics:
(a) Will not leak or deform unduly under test pressures; PA1 (b) Resists angular deflections in the pipes to be joined, but allows some flexibility; PA1 (c) Is strongly resistant to deformation due to lateral shear forces from pipe misalignment; PA1 (d) Can resist pull-out forces due to internal test pressures; PA1 (e) Has a long service life; PA1 (f) Is easy to assembly and install; PA1 (g) Holds the pipes in a concentric alignment; and PA1 (h) Is economical in use and is cost effective.
The soil pipe coupling set outin the standards and in common use today has consistently out-performed all others in generally fulfilling the above requirements.
The soil pipe couplings commonly used comprise, in general, three main elements. First, there is a sealing element, made of relatively soft, resilient elastomer of hardness 35 to 70 on the Shore Durometer A Scale. The sealing element is tubular and is normally stretched over the two pieces to be joined to form a leak-proof joint when clamped. Secondly, a corrugated shear ring, usually of stainless steel is clamped over the sealing element and provides the strength to resist deformation due to shear forces which would otherwise offset pipes or fittings at the joint, producing increased flow resistance inside the pipe. Thirdly, clamping means apply the necessary pressure to the sealing element to make the joint leak-proof and supply rigidity to the coupling when applied in conjunction with the shear ring. These clamping means are normally in the form of stainless steel hose clamps but cast iron segments bolted together with stainless steel bolts are also used.
Both the United States and Canadian Standards allow the outside diameter of cast iron pipe and fittings to vary approximately 1/4 inch. The original coupling designs, which are still in use, use a thin elastomeric gasket which is sized so that it will slip over the minimum diameter. This gasket must be stretched over any other diameter and thus is normally under tension when applied to a pipe or fitting. On maximum diameters or during cold weather, when the elastomeric gasket becomes very stiff it can be difficult to assemble a joint. It is often necessary to heat the gasket so that it can be stretched over the pipe end.
Because the gasket must be stretched over the pipes to be joined, it is necessary to separate the clamping means from the gasket and then to apply the neoprene gasket separately. The clamping means must then be applied over the gasket. Thus two items must be separately applied to complete a sealed joint. This adds considerably to the time and effort required to assemble a joint.
Because the hose clamps are rivetted or welded to the corrugated shield, the clamping means are essentially one piece. To achieve a satisfactory joint, the clamps on either side of the joint must be progressively tightened, alternating on each side until the hose clamps reach their proper tension. Because of this procedure, in practice it is impossible to connect this coupler to one pipe and then complete the connection to the next element in the plumbing system at a later time. Subassembling is thus inhibited.
Furthermore, when connecting two pipes or fittings of different diameters, this coupling will cause one side of the matching parts to align. So that all the difference in diameter occurs on the opposite side, that is the joint is not concentric. This can result in dams and other flow impediments.
The shortcomings outlined above are present in the standard couplings used today.